Electrolytic device



Patented Mar. 24, 1936 ELECTROLYTIC DEVICE Preston Robinson, Williamstown, and Joseph L. Collins, North Adams, Mass., assignors to Snr zue Specialties Company, North Adams, Mass., a corporation of Massachusetts No Drawing. Original application Mam." 28,

1931, Serial No. 526,118. Divided and this application January 11, 1935, Serial No. 1,432. In Canada August 4, 1932 6 Claims. (01. 1'75-315) Our invention relates to electrolytic devices, acidic bath and subsequently rinsing them in densers-otherwise such as electrolytic condensers, rectifiers, or the like, comprising film-forming electrodes, this application being a division of our copending application Ser. No. 526,118, filed March 28, 1931.

In such devices use is made of the film-forming effect exhibited by certain metals, for instance aluminum which, when placed in suitable electrolytes, upon application of the proper volt age are covered with a film. Such films consist of partially hydrated aluminum oxide and have a uni-directional character.

We shall describe our invention in its application to electrolytic condensers, using aluminum electrodes, although it should be well understood that our invention is not limited to such devices or electrode material.

We .have found that the oxide layer which forms on aluminum, when exposed to the action of acids or to air, while chemically similar to the substance of the film built up in the electrolytic forming process, substantially diifers therefrom in its insulating and dielectric properties. We have also found that if an aluminum electrode, before it is subjected to electrolytic formation, is covered by such an oxide layer, such layer disadvantageously influences film formation, respectively, the properties of the films obtained. More specifically we have found that condensers having electrodes, which were thus oxidized prior to their formation have higher power factor and smaller effective capacity than conidentical-in which trodes are used on which oxidization before film formation has been prevented.

It is, therefore, one object of our invention to provide films of 'improved qualities by preventing oxidization of the electrode surface prior to the formation of the film.

Other objects of our invention will appear as the specification progresses.

As in the manufacture and handling of the electrodes impurities such as filings, grease, etc., are deposited on and adhere to the electrode surface, the deleterious influence of which is well recognized, it is practice to clean the electrodes before subjecting them to the film-forming process.

In the-past, such cleaning was effected by subjecting the aluminum electrodes to an aqueous water. However, the acid while removing the impurities from the surface of the aluminum, also dissolves the outer layer of aluminum, and-. due to the water present-forms an oxide layer 5 on the electrode. This oxide layer is not removed through rinsing and its presence, as stated above, deleteriously aflects the quality of the dielectric film.

We have found that by immersing the aluminum in a properly prepared alkaline solution, we can remove the impurities just as effectively as with an acidic cleanser without forming an oxide layer on the aluminum.

Attempts to use alkaline cleansers for such purpose, have been unsuccessful so far, because an excessive amount of alkaline material was required to properly clean the electrode. In addition, the alkaline material attacked and pitted the aluminum to such an extent, particularly around metal impurities in the surface, that it was unfeasible to form a good quality film on the electrodes so cleaned, particularly in the neighborhood of the exposed impurities.

To prevent pitting of the aluminum, it has been suggested to treat the anodes in a weakly alkaline solution such as borax or trisodium phosphate which will attack aluminum oxide or hydroxide, but which will not strongly attack aluminum. However, in order to obtain complete cleaning with such reagents, it would be necessary to operate at very high temperatures and for long continued times, which is objectionable and uneconomical.

We have found that the pitting action of a strongly alkaline solvent can be overcome, respectively changed in character, by adding to such solventwhich for instance may be caustic sodaa suitablev inhibiting agent, such as sodium phophate, sodium meta-silicate, sodium fluoride, etc. The strong local action of the alkali is thereby transformed into an evenly distributed moderate action and the aluminum, instead of being pitted and damaged in spots, is evenly etched over its whole surface. At the same time, only a small amount of caustic reagent is required.

Thus by using a caustic cleanser in the presence of proper inhibiting agents, objectionable 'cxidization of the electrode is prevented, and at the same time through evenly distributed etching of the aluminum surface, the effective area of the electrode is considerably increased.

After their cleaning in a caustic solvent, the electrodes are preferably subjected to rinsing.

However, we have found that objectionable Oxidization of the electrode before it is formed is not fully prevented even by alkaline cleaning.

because, as above stated, the cleaned electrode when left exposed to air, oxidizes on its surface and this oxide layer is just as objectionable as the oxide film'formed in an acidic cleaner.

Even relatively short exposures to air of the cleaned electrodes cause a marked increase in power factorand decrease in capacity, and such deleterious effects increase ,with the duration of such exposure; and we found that the lowest power factors and maximum capacities are obtained when the electrodes, after their cleansing and rinsing are immediately submerged in the forming electrolyte and subjected to formation therein. 1

For instance, taking two electrodes of identical construction and cleaned by the same process and leaving one before its formation for half an hour exposed to air, while subjecting the other to the forming process immediately after its cleaning,

we have found the following: The capacity of the condenser using the first electrode was 6.2 mfd. and its power factor 24%,

while the condenser using the second electrode the capacity was 7.1 mfd. and the power factor 15%.

In practice, therefore, after we have cleaned the electrodes in a proper alkaline solution and subsequently rinsed them preferably in a very weak alkaline solution-as a diluted solution of borax-we transfer the electrodes from the cleaning bath immediately into the forming tank and subject them therein immediately to formation, whereby the electrolyte used inthe formation process is preferably slightly acidic.

Instead of using the above described method, we have found that undesirable oxidization of the electrode can be prevented by a second method as follows:

The electrodes, instead of being subjected to a preliminary chemical cleaning, undergo a preliminary forming process, which consists of placing the electrodes in a slightly acidic electrolyte, for instance, borax and boric acid in which the electrodes are subjected to a short formation. As a result of this formation, a thin oxide film covers the electrode, which film also includes the surface contaminations of the aluminum.

After such preliminary formation, the electrodes are submerged in an alkaline solution, for instance, borax, sodium phosphate, etc., which attacks and removes the preliminary film without oxidizing or attacking the aluminum. Thereafter; the electrodes are subjected to the regular forming process, preferably in a slightly acidic electrolyte.

In using this method, it is sometimes found advisable to have the alkaline solution only dissolve such outer portion of the preliminary film which actually contains the impurities, and to leave the inner portion of the film adhering to the electrode.

A third method of obtaining the same or siml lar results is the following:

The electrode, without preliminary cleaning,

is placed in the forming tank which contains an alkaline electrolyte, for instance, borax. Current is now applied, whereby a film is formed on the electrode but partly attacked by the borax, Hereafter the electrolyte is made slightly acidic by addition of boric acid and the electrode subjected to the regular formation process.

A fourth method is similar to the third and consists in placing the electrode without preliminary cleaning in the forming tank, which again comprises an alkaline solution as caustic soda, but instead of applying a forming current, the caustic soda is left chemically to clean the aluminum-preferablyin the presence of inhibitive agents-as described in connection with the first method. After a short. period of alkaline cleaning of the electrode, the electrolyte is made acidic, for instance, by addition of boric acid and the forming process undertaken in the regular way.

We have also found that while the highest quality commercially available aluminum, the so-called grade A" aluminum, having 99.6% to 99.7% aluminum is subjected to corrosion in air,

an aluminum of 99.9% or higher purity, does not special precaution is required to prevent the elec-- trode from corroding in air before it is subjected to formation.

In the foregoing we have described various methods to prevent objectionable oxidation of aluminum electrodes during or subsequent to their cleaning and to obtain higher quality oxide films. While we have illustrated our invention in connection with electrolytic condensers, using aluminum electrodes and described specific methods, we do not wish to be limited to such devices, material or methods, but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What we, therefore, claim and desire to secure by Letters Patent is:

1. In the manufacture of film-forming electrodes for. electrolytic condensers, the process which comprises subjecting the electrode to a preliminary forming process in a slightly acidic electrolyte, dissolving the film so formed in an alkaline solution and subjecting thereafter the electrode to a' final film formation.

2. In the manufacture of film-forming elec trodes for electrolytic devices, the process which comprises subjecting the electrode to a preliminary fllm formation in an electrolyte, dissolving the outer portion of the film by immersing the electrode in an alkaline solution and subjecting the electrode thereafter to a final forming process.

3. In the manufacture of an aluminum-electrode, the process which comprises subjecting the electrode to film formation in an electrolyte comprising an alkaline salt of a weak acid and subsequently adding an excess of a weak acid to the electrolyte and completing film formation in the acidic electrolyte.

' 4. In the manufacture of aluminum electrodes nary film-formation in an electrolyte, dissolving the outer portion of the film and subjecting the electrode thereafter to a final forming process.

6. In the manufacture. of film forming electrodes for electrolytic devices the process which comprises, subjecting the electrode to a preliminary film formation in an acidic electrolyte, Greating the electrode in an electrolyte having a pH higher than 7 and subsequently placing the electrode in an electrolyte having a pH lower than 7 while applying to the electrode a voltage-effecting film formation.

PRESTON ROBINSON. JOSEPH L. COLLINS. 

